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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Accounting Policies [Abstract]
Lease revenue	$ 55.0	$ 54.0	$ 109.9	$ 108.5
Current and non-current deferred credit balances	14.7		14.7		$ 15.4
Recognized deferred credits to revenue	1.4	1.2	2.6	2.2
Contract assets balances	5.0		5.0		$ 4.6
Contract cost amortization	0.6	0.5	1.2	1.0
Engine overhaul expense	5.2	13.7	9.4	33.1
Engine overhaul pass-through expense	(0.4)	2.9	1.2	5.2
Airframe check expense	4.7	7.9	6.2	13.5
Airframe check pass-through expense	$ 0.0	$ 2.7	$ 0.0	$ 5.7

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Airframe check expense.

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Amount of amortization expense for asset recognized from cost incurred to obtain or fulfill contract with customer.

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Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time.

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Amount of operating lease income from lease payments and variable lease payments paid and payable to lessor. Includes, but is not limited to, variable lease payments not included in measurement of lease receivable.

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